Pierogi Ruskie (Potato and Cheese Pierogi)

Name: Pierogi Ruskie (Potato and Cheese Pierogi)
Cook time: 1 h 30 min
Total time: 1 h 30 min
Cuisine: European
Ingredients: 2 cups/255 grams all-purpose flour (preferably unbleached), plus more as needed; 1 teaspoon kosher salt; 3 tablespoons unsalted butter; 1 large egg, beaten; 1/2 pound waxy or all-purpose potatoes; Salt and pepper; 3 medium yellow onions (about 8 ounces each), finely chopped; 1/2 cup quark cheese, cottage cheese or sour cream (about 4 ounces); Butter, for pan-frying (optional); Sour cream, for garnish; Chopped fresh parsley or dill, for garnish